Bison subspecies - American Bison Society

Bison subspecies Matthew A. Cronin University of Alaska Fairbanks School of Natural Resources and Agricultural Sciences Palmer Research Center

Subspecies taxonomy • Zink (2004): “Mitochondrial DNA sequence data reveal that 97% of …avian subspecies lack the population genetic structure indicative of a distinct evolutionary unit… the subspecies rank will continue to hinder progress in taxonomy, evolutionary studies and…conservation.”

Subspecies definitions • Avise and Ball (1990): “Subspecies … phylogenetically distinguishable from, but reproductively compatible with other such groups…phylogenetic distinction must … come from the concordant distributions of multiple, independent, genetically based traits.”

Phylogeny is the basis for subspecies classification

• This is the critical criterion for taxonomy

Phylogenetic inference • Morphology • Biogeography • Paleo studies-fossils • Molecular phylogeny

Caveats • Gene trees are not equal to (sub)species trees • mtDNA has been the primary gene used in intraspecific phylogenies

Examples of discordant mtDNA and species trees • Bears • Deer

mtDNA of Bears ************************************ Black Bear MT and OR ******* • * ************************************ Black Bear MT and OR •

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**************************** Alaska and Canada * Grizzly Bear * • * * • *************** **************** Southeast Alaska ABC Islands * * Brown Bear • • *********** * • **************** Polar Bear

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0.08

0.05

0.03

Base substitutions/nucleotide

0.01

So gene (mtDNA) trees may not be concordant with species or population trees This is a dilemma when taxonomy is based on phylogeny

Wood bison Alaska

Wood bison Alaska

Plains bison

http://www.cryptomundo.com/wp-content/uploads/p4.gif

Wood Bison Range

Environmental variance

Bison subspecies Do not have monophyletic mtDNA Cronin 1986, 1993, Polziehn et al. 1996, Douglas et al. 2010

• Bison subspecies • Microsatellite DNA genetic distances larger between than within the subspecies • Wilson and Strobeck 1999

Wood bison and Plains bison Have some (heritable?) morphological differences Do not have monophyletic mtDNA Share nuclear k-casein alleles with different frequencies Share microsatellite alleles with different frequencies. May have had contiguous ranges and gene flow prior to drastic reduction in numbers in the 1800’s

Extant herds of wood bison have had mixing with plains bison Some extant plains bison herds have cattle genes resulting from hybridization

Data do not support wood and plains bison subspecies • Polziehn et al. 1996 (mtDNA): “…neither subspecies of bison is a well-defined taxon” • Douglas et al. 2010 (mtDNA): …”(wood and plains bison) should not be considered subspecies” • Geist 1991 (morphology):”…The wood bison is not a valid taxon” • Cronin and Cockett 1993 (k-casein): “No fixed allelic differences were observed…among bison herds or subspecies”

Data support bison subspecies Morphology Microsatellite DNA genetic distances and k-casein alleles suggest some differentiation of allele frequencies Wilson and Strobeck 1999 Microsatellite

DNA genetic distances larger between than within the subspecies

Extant herds a taxonomic problem Extant wood bison herds have mixed with plains bison

Wilson and Strobeck 1999 Halbert et al. 2005

Does it matter? Plains bison survive in Alaska and plains X wood bison herd survives in Canada

Maintaining fit populations with adequate genetic variation can be done without concern over maintaining pure bison subspecies

A word on Terminology: “Tyranny of the Discontinuous Mind”(Dawkins 2005) • • • •

Chronospecies Subspecies Population Subpopulation

Wood bison subspecies on the ESA list is preventing (re) introduction of wood bison to interior Alaska